Increased ventilatory demand | Increased physiologic dead space | Pulmonary embolism | Extrinsic compression of pulmonary artery | Emphysema | Metabolic | Exercise | Altered pCO2 set point | Early metabolic acidosis due to deconditioning | Neurohumeral | Pain | Anxiety | Depression | | Impaired mechanical response | Airway obstruction | Chronic bronchitis | Tumor | Decreased chest wall compliance | Post-thoracotomy | Obesity | Decreased parenchymal elasticity | Pulmonary fibrosis | Pulmonary vessel congestion secondary to left heart failure | Inspiratory muscle weakness | Neuromuscular disease | Cachexia due to cancer | Steroid myopathy | Electrolyte imbalance | |